AIDS retrovirus grows to very high titers in a subclass T lymphocytes and the fatal outcome of the disease is thought to be consequent upon selective ablation of a subclass of T lymphocytes. In addition to the usual retroviral gag, pol and env genes, the AIDS RV possesses several additional open reading frames (ORFs) and further augments its coding potential by using complex splicing schemes of mRNA biogenesis. To understand the mechanism of its gene regulation, we have analyzed the viral transcripts by RNA filter blotting and hybridization, nuclease S1 mapping of the splice junctions and cDNA cloning of viral mRNAs. Three major classes of subgenomic RNAs (5.5, 5.0 and 4.3 kb) were found to possess a 289 base leader spliced to the 3' portion of the genome extending from positions 4542, 4914, 5650, and/or 6100, respectively. A fourth class of mRNA had a variable (1.7-1.9 kb) exon at the 3' end spliced possibly via one or two internal exons to the leader RNA. An additional small splice junction identified near the end of the gag ORF was thought to generate gag-pol fusion mRNA. To determine the functional potential of the different viral transcripts, we have constructed a library of cDNA expression plasmids representing viral RNAs in eukaryotic vectors designed by OKayama and Berg. From this library, we have isolated several unique clones corresponding to differentially spliced class 4 viral mRNAs. By DNA transfection, these plasmids have been introduced into cos cells and the unique viral gene products synthesized in these cells are in the process of being identified by immune detection.